1. Field of Invention
This invention relates to drive circuits for gas discharge lamps. More particularly, this invention relates to a fluorescent lamp power supply circuit that uses a multi-layer piezoelectric acoustic transformer to produce a high-frequency, high-voltage excitation output to drive the lamp.
2. Description of Prior Art
All gas discharge lamps, including fluorescent lamps, require ballasts to operate. The ballast provides a high initial voltage that is necessary initiate the discharge, then rapidly limits the lamp current to safely sustain operation.
There are two general types of ballasts, classified based on the kinds of electrical components used to build the ballast: Magnetic ballasts and electronic ballasts.
Magnetic Ballasts consists of a transformer and a capacitor encased in an insulating material. In 1990 U.S. energy standards began requiring that standard magnetic ballasts no longer be sold, although these older ballasts still make up the majority of fluorescent ballast installations. Magnetic ballasts sold after 1990 bear the designation "energy-saving", and are about eight percent more efficient than older ballasts. The lamp operating frequency is 60 Hz, the same as the input frequency. Electronic ballast have many advantages over magnetic ballasts, including reduced flicker, noise, heat, size and weight, and increased lighting efficacy, high power factor and low total harmonic distortion (THD).
Electronic Ballasts use a variety of electronic components to convert the lamp operating frequency from 60 Hz to 20-40 kHz. The high frequency operation reduces internal losses in the lamp and conserves energy.
FIGS. 1 and 2 illustrate two exemplary prior electronic ballast circuits. FIG. 1 illustrates a prior ballast circuit comprising an oscillator IC (U1) and three field effect transformers (Q1, Q2 and Q3), among other components, such as may typically be used in a 24 watt fluorescent lamp ballast. FIG. 2 illustrates a prior ballast circuit comprising a pair of field effect transformers (Q1 and Q2) and a step-up electromagnetic transformer T1, among other components.
An overview of the operation of these two prior ballast circuits is useful in order to appreciate the advantages that the present invention provides.
Referring first to FIG. 1: FIG. 1 schematically illustrates a typical prior ballast circuit as may be used to power a 24 watt compact fluorescent lamp 50. In FIG. 1 the lamp 50 has cathode connections J3, J4, J5 and J6. A 60 Hz alternating current input AC provides power to the circuit. Capacitor C7 is provided to reduce EMI from escaping the circuit and going back up to the AC line. The capacitor C7 essentially opens the circuit for 60 Hz operation, but it shorts for a frequency of about 60 kHz, which is the frequency at which this circuit normally operates. The rectifier bridge BR1 produces DC voltage at about 170 volts, nominal. This voltage is not ground referenced, it is a floating reference such that there is a 170 volt difference from one side of the circuit to the other.
Coming out of the bridge rectifier BR1 is a 47 microfared 200 volt capacitor C1, which reduces the ripples in the rectified current. A 100 uH inductor L1 is provided to isolate the high frequency current pulses that are being drawn from the positive rails. Resistor R1 (270K) provides a small amount of current to start oscillator U1. The oscillator U1 generates gate control voltages for the field effect transformers Q1 and Q2 (FETS) that are connected to pins 5 and 7, which are marked HV (high voltage gate) and LVG (low voltage gate), respectively.
The low voltage gate (LVG) line (pin 5) is connected to FET Q2, which can be driven with only 10 to 15 volts at the gate. On the other hand, the source of FET Q1 is connected to the output signal of FET Q2, so that it is necessary to be 10 or 15 volts above that source value. When FET Q1 is turned on the voltage on the output line (pin 6) rises up to the rail voltage of 170 volts. Oscillator U1 pin 8 (marked "boot"), generates internally high "boot strap voltage", which can go above the rail voltage, so that the gate of Q1 may become turned on when it goes up to the high voltage supply. A square wave comes out of pin 6 of the oscillator U1 and is coupled back to capacitor C10. That pulsing square wave enables the generation of a high voltage which is used to drive the gate on the high side FET Q1.
Resistors R3 and R4 are located at the gates of the two transistors Q1 and Q2. One series gate resistance prevents the transistors Q1 and Q2 from self oscillation at very high frequencies, (which could be destructive, as its generates a lot of EMI). This resistance has a tendency to slow down the rise time somewhat, which reduces EMI.
Capacitor C3 and inductor L2 are in series with cathode 1A of the lamp. The inductor L2 is composed of a parallel dual winding on a single core. Parallel dual winding is used to reduce the skin effects at high frequency. Skin effect is predicted by Lens Law which states that in a current carrying wire, the thicker the wire the larger the self induced current. That self induced current has a tendency to travel in the opposite direction down the center of the wire. Accordingly, if the cross section of the wire is minimized, the surface area of the wire is maximized, which permits the maximum forward current, the minimum reverse current. Thus impedance at high frequencies (i.e. series resistance) is reduced, which would result in high losses if the skin effect weren't reduced. Typically such prior circuits use wire comprising many fine strands. Capacitor C3 and inductor L2 form a series resonance circuit. A series resonance circuit can be used in this manner to generate a large voltage swing such as is typically required in prior fluorescent lighting circuits in order to "fire" the lamp. When that series resonance circuit is driven at its resonant frequency, it generates a high voltage that will fire the lamp.
Resistor R5 and capacitor C13 at the output of the transistors Q1 and Q2 provide a "snubber circuit". The series resonance circuit, which comprises a large inductor L2, can result in "overshoot" and "ringing" unless a snubber circuit is used to prevent such effects, e.g. by dissipating a small amount of energy into the air and by reducing the rise time of the voltage so as to help control very high frequency EMI. Some of the energy is picked off from the snubber circuit runs through capacitor C5, diode D2 and diode D1, and back over to pin 1. When this circuit is running it requires significantly more current to drive the gates at this frequency, because these gates are capacited and it is necessary to charge and discharge them. So, if resistor R1 were low enough value to supply the gate current that is required from 170 volts to bring it down to 15 volts that is internally regulated on this device, it would dissipate a lot of energy (on the order of 2 watts) in resistor R1. However, since there essentially exists a miniature switching power supply in capacitor C5, diode D2 and diode D1, once the circuit starts, it uses the switched output voltage to generate the current necessary to keep the device running. During start up, capacitor C2 is initially brought up to voltage by resistor R1 and it supplies enough current during start up to run the gates. After a cycle or two of operation, currently is already being supplied back from the feedback circuit, the circuit/lame can continue to run.
Capacitors C8 and C6, along with resistor R10 form a capacitive type voltage divider, past the resistive voltage divider. R10 is only 1 OHM. There is a 1 ohm difference between the low voltage sides of capacitors C8 and C6 (which are connected to the source of FET Q3) and the ground reference for the oscillator U1 at pin 4. Part of the energy being used to drive the circuit in the lamp is, in this manner, fed back to FET Q3.
Field effect transistor Q3 starts in the "off" position. The drain of FET Q3 is connected to the junction of capacitors C9 and C11. The series combination of capacitors C9 and C11 is attached to oscillator pin 3, which is also attached to resistor R2. The R-C time constant of resistor R2 and the series capacitance (C9 and C11) determines the operating frequency. The capacitance (at pin 3, "CF") and the resistance (at pin 2, "RF) are used to set the frequency. So, initially when the circuit starts running, resistor R2 and the combination of capacitors C9 an C11 control the frequency. That frequency is not at the resonant frequency of the resonant circuit; and, because it is not at the resonant frequency of the resonant circuit, the voltage that the lamp sees is lower then it sees at the resonance frequency. However, attached to junctions J3 and J4 is a filament. So, the frequency initially seen by the filament between junctions J3 and J4 is lower frequency than the running frequency. The current at that (lower) frequency goes out of junction J3 and, not through the lamp tube to junction J6, but through the filament over to J4, thence through the capacitor C14, back out of junction J5 through the second filament to junction J6.
During that phase of start-up, the filaments begin to heat, and this heating condition continues as long as the tube is not fired. So, during this start-up phase, capacitor C12 has virtually no charge when the power is first turned on, and it begins charging through resistors R1 and R6, and pin 1 of the oscillator U1 is maintained at 15 volts. The 15 volts applied to R6 charges capacitor C12. When capacitor C12 charges up to 10 volts plus the gate turn-on voltage. A zener diode D3 is used to block any of this charge-up voltage from getting to the gate of FET Q3 until the voltage is above 10 volts. The gate voltage at FET Q3 of about 2.5 volts is necessary to turn FET Q3 on. Resistor R7 insures that the gate of FET Q3 is pulled low and it just drains off any leakage current, so that once capacitor C12 charges up to about 12.5 volts, this FET Q3 turns on.
When FET Q3 (which has a resistance of about 1 ohm) turns on, it shorts out capacitor C11. When capacitor C11 is shorted out, the RC time consonant is determined solely by capacitor C9 and resistor R2. Since FET Q3 only has about one ohm resistance, its has little effect because the resistance of resistor R2 (which is on the order of 10.7K OHMS resistance) is so large.
With the filaments warm, as the frequency shifts the voltage rises at the lamp because the circuit is now running capacitor C3 and inductor L2 at resonance. Once the lamp fires, the voltage at the lamp across junctions J3 and J6 is typically up around 6 or 7 hundred volts peak-to-peak. Once the lamp fires it runs at about 80 volts RMS, or slightly 200 volts peak-to-peak. Now the capacitor filament series circuit (comprising capacitor C14), instead of seeing the higher voltage of approximately 350-400 volts peak-to-peak of the start up phase, is now only seeing a little over 200 volts peak-to-peak during the normal running phase. During normal running phase of the circuit the filaments no longer receive electric current to keep them warm; but, because the tube is fired, the plasma in the tube keeps the filaments hot.
In a hot cathode tube such as is used in this circuit, the goal is to heat up the filaments and, typically, they are required to be heated three to four times their initial resistance. In other words, at start up the filaments are heated up until their resistance is about 4 times their initial resistance. When the filaments begin to glow, the coating on the filaments emit electrons. This emission of electrons into the space of the tube reduces the voltage required to fire the tube. If a higher voltage or an electric field from an external trigger next to the tube is then applied, the atoms in the tube can be excited to a level where they can conduct. The conduction voltage depends on the vapor pressure of the mercury and temperature inside of the tube.
When the tube begins to conduct, the voltage between the two ends of the tube drops to the reduction voltage. The voltage at the filaments becomes much lower and, accordingly, their self heat is not as great. Once fired, the high temperature of the plasma in the tube is sufficient to keep the filaments warm, and, accordingly, little energy (on the order of 0.7 watts) is dissipated while the lamp is running.
The circuit depicted in FIG. 1 is typical of a 24 watt lamp/ballast. This circuit typically achieves about 85% efficiency. That is: the circuit draws about 24 watts, approximately 20.5 of which go to lighting the lamp.
Referring now to FIG. 2: FIG. 2 schematically illustrates a second type of prior electronic ballast for a fluorescent lamp. The circuit shown in FIG. 2 is a 15 watt circuit. Input to circuit (110 VAC, 60 Hz) is the same and rectified as described with respect to the circuit of FIG. 1.
This prior circuit has an N-channel FET Q1 and a P-channel FET Q2, whereas the circuit of FIG. 1 has two N-channel FET. The advantage of using an N-channel and a P-channel FET is that the two sources can be connected together. N-channel FETS (such as are used in the circuit of FIG. 1) are better for current and speed, but they much be connected starting from the high voltage supply down, so that the transistors are "stacked" drain, source, drain, source. In the circuit of FIG. 2, starting at the positive rail and going toward the negative rail, the FETS (Q1 and Q2) are arranged drain, source, source, drain.
In the circuit shown in FIG. 1, the ground reference internally in the circuit is on the negative side; so, the circuit operates off of 100 plus 170 volts. In the circuit shown in FIG. 2, virtual ground is generated by using a series combination of capacitors C2 and C3. The value of these capacitors C2 and C3 is about 0.22 uF. By connecting the ground between these two capacitors C2 and C3, the AC signals can swing (positive) above that ground and (negative) below that ground. This approach differs from the circuit shown in FIG. 1, in which circuit the ground reference can only be driven upward in the positive direction. The plus/minus supply scheme of the circuit shown in FIG. 2 enables the use of a combination of N-channel and P-channel FETS (Q1 and Q2), which simplifies the gate drive.
Field effect transistor Q2 is a P channel device, and FET Q1 is an N-channel device. Resistor R3 is connected attached to the positive voltage rail. When the circuit is first turned resistor R3 causes the voltage on the gate of the FET Q1 to rise until it turns on. When FET Q1 turns on, the voltage at its output line, which is connected to transformer T1, begins to rise, and continues to rise to the voltage of the positive rail.
The primary coil of transformer T1, which typically has about 100-102 turns, acts as an inductor. This "inductor" forms a series resonance circuit with capacitor C9 and the lamp.
The current through the secondary (6 turn) side of transformer T1 goes through inductor L2 and capacitor C5, (which is a series resonance circuit back), and back to the gates of the FETS Q1 and Q2. After the current reaches a peak, it begins to decay. As the magnetic fields decays this reverses the current flow through the inductor L2, which then begins to pull the gate voltage in the opposite direction.
Because resistor R1 is high resistance (270 kOhm) little current passes to the gates of FET Q1 or Q2. Thus, the gates of the FET Q1 and Q2 are essentially open when there is a strong AC signal, and this pulls the gate voltage down. In doing so, the voltage swings in the negative direction. Now FET Q2 turns on and FET Q1 turns off. The charge that was in the capacitors C9 and C6 attached to the lamp now begins to discharge through the transformer T1 to the minus side. When capacitors C6 and C9 have discharged, the voltage swings the other way, and the opposite condition exists. This, in turn, causes FET Q1 to turn on and FET Q2 to turn off.
Inductor L2 and capacitor C2 are set up to the same frequency as the primary of transformer T1 and the capacitance attached to the lamp. Additionally, it is noted that in this circuit the sources of the FETS Q1 and Q2 are connected together on the output line. Two zener diodes D1 and D2 are attached back to back to the source line. At any given time, one of the diodes (D1 or D2) will be forward biased and the other will be reversed biased.
The gate to source voltage determines whether an FET is turned on or off. However, that voltage must not exceed the gate source rating. The FETS typically used in prior circuits such as these are typically rated in the 15 to 20 volt range. The two zener diodes D1 and D2 are connected in series. The zener diodes are forward biased at approximately 0.7 volts. When the diodes are reverse biased they break down, yielding avalanche voltage at the zener voltage, which, in this case, is 10 volts. So, this combination of back to back zener diodes clamps the voltage that the gates of the FETS Q1 and Q2 at about 11 volts. This prevents the inductor-capacitor voltage from swinging too high, so as to prevent overheating of the gates of the FETS. A voltage divider is not used in this circuit because the differences between the start-up voltage and the running voltage would be too high, which would result in too much variation in the gate voltage. Instead, the voltage is allowed to run high and then is "clamped" with the diodes D1 and D2.
Resistors R4 and R5 in series with the gates of the FETS Q1 and Q2. Resistors R4 and R5 are basically for the same purpose: namely, they slow down the rise time and they prevent self oscillation.
Positive temperature coefficient resistor RT1 and capacitor C6 affect the resonance frequency slightly, so that the voltage is limited across the lamp. The PTC RT1 has a low resistance (approximately 300 ohms) at room temperature.
When the circuit is initially turned on, current flows through PTC RT1 and through the lamp filaments. As PTC RT1 begins to heat up it resistance increases rapidly. At the same time, the lamp filaments are also heating up. When the resistance of PTC RT1 gets high enough not to load down the circuit, the resonant frequency shifts because capacitor C6 becomes isolated from the rest part of the circuit. Then the voltage of transformer T1 and capacitor C9 begins to rise across the lamp. The filaments, now subjected to the high voltage, become hot, and the frequency begins to change more toward the resonant frequency and the lamp fires. Then once the lamp fires there is a much reduced voltage across the filaments and PTC RT1. There will still be some current flow through the PTC, but, because the PTC resistance has risen, it is able to dissipate most of its heat. The self heating of the PTC keeps its resistance in equilibrium at high level.
It will be noted that, once fired, although most of the current flows through the lamp filament, there is always a loss associated with current flow through the PTC.
Recent government regulations dictate the amount of EMI emissions that are allowed from fluorescent lamp ballasts, as well as the extent to which circuitry must provide lamp removal protection.
In view of the foregoing discussion, it would therefore be desirable to provide a power-supply and control circuit for a gas discharge lamp that uses a device with inherent fundamental characteristics that are more conducive to the device being used as a step-up transformer than the characteristics of a conventional magnetic-coil transformer.
It would also be desirable to provide a power-supply and control circuit for a gas discharge lamp that uses a device with inherent fundamental characteristics that are more conducive to the device being used as a step-up transformer than the characteristics of a conventional Rosen-type piezoelectric transformer.
It would also be desirable to provide a power-supply and control circuit for a gas discharge lamp using a step-up transformer that can be inherently made smaller in all dimensions than conventional magnetic transformers.
It would further be desirable to provide a power-supply and control circuit for a gas discharge lamp that can be made smaller than conventional circuits using either magnetic step-up transformers or prior piezoelectric transformers.
It would additionally be desirable to provide a power-supply and control circuit for a gas discharge lamp having a simpler construction than conventional, magnetic or piezoelectric, power-supply circuits.
It would still further be desirable to be able to provide such a gas discharge lamp power-supply and control circuit that uses a step-up transformer that does not require a sinusoidal input in order to generate a high-frequency, high-voltage AC output.